Electrical measuring instrument.



No. 634,965. Patented 00L l7, I399.

E. THOMSON.

ELECTRICAL MEASURING INSTRUMENT.

(Application filed Dec. 31, 1897.)

(No Model.)

Wc'f/pesses TNE NORRIS PzYsns co. PHOTO-LITYHQI, WASNYNGTON. n. cy

' a spring and in others by a weight.

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ELIHU THOMSON, OF SWVAMPSCOTT, MASSACHUSETTS, ASSIGNOR TO THE GENERALELECTRIC COMPANY, OF NEIV YORK.

ELECTRICAL MEASURING INSTRUMENT.

SPECIFICATION forming part of Letters Patent No. 634,965, dated October17, 1899. Application filed December 31, 1897. Serial No. 664,980. (Nomodel.)

To (tZl whom it may concern:

Be it known that I, ELIHU THOMSON, a citizen of the United States,residing at Swampscott,county of Essex, State of Massachusetts, haveinvented certain new and useful In1- provements in Electrical MeasuringInstruments,of which the followingis aspecification.

In electrical instruments indicating volt and ampere meters, forexample, the mechanism is arranged in such manner that current flowingthrough one or more of the parts will deflect an indicator over agraduated scale. The movement of the indicator has heretofore beenopposed in some instances by In a form of commercial instrument commonlyused a permanent field magnet is employed, and within the field of themagnet mounted on 3' ewel-bearin gs is a swinging coil of wire which istraversed by current from the circuit to be measured. Electricalcommunication with the movable coil has heretofore usually beenestablished through coiled springs, which are connected to the shaft ofthe coil and some stationary part of the instrument and are adapted tobring the coil and indicator back to zero position when no current isflowingin the coil. This arrangement is not entirely satisfactory owingto changes in the strength of the magnet or magnets and the liability ofthe parts being damaged, thus changing the zero position of theinstrument and also the amount of deflection for a given current orvoltage. The greatest difficulty is occasioned by variation in thestrength of the magnets. Such variations may be caused by heavy currentspassing near the instrument, which set up stray fields influencing themagnets. The magnetism also slowly decreases as the magnets become aged.Such variations,whatever their cause, introduce error in the reading ofthe instrument in direct proportion to the change of strength inthemagnets. Another type of instrument has been devised in which afield-magnet is employed to deflect the moving-coil system and aseparate magnet, either permanent or electro, for opposing this movementand returning the moving system to zero. This type of instrument,however, is

not satisfactory, because since the moving and return systems are actedupon by different fie1d-1nagnets a change in one system,due to a changeof field strength, does not pro duce a corresponding change in theother.

To overcome the objections pointed out, to render the instrument moreaccurate, and to simplify its construction, I propose to employ a movingcoil through which the whole or a portion of the current to be measuredpasses and mount this coil within the influence ofa magnetic fieldproduced either by a perma' nent or electro magnet. To return themoving-coil system to zero, or, in other words, to furnish the restoringtorque of the instrument, I employ what I term a magnetic return-stripand secure it in place in such manner that it is acted upon by the samefield-magnet that produces a deflection of the moving-coil system. Therelation between the coil system and the return-strip is such that theneedle of the instrument is brought to rest at different pointscorresponding to different values of the current to be measured. Inother words, the force exerted by the moving coil opposes that of themagnetic returnstrip. The moving system sets itself at a point Wherethese forces balance, and. the index or needle indicates the degree ofdeflection and the amount of current or the voltage across the terminalsof the instrument.

In the accompanying drawings is shown an embodiment of my invention, inwhich- Figure l is a plan view of a measuring instrument with theinclosing case shown in dotted lines. Fig. 2 is a side elevation showing the casing and a portion of the base in section, and Fig. 3 is asection on line 3 3 of Fig. 1.

The base A is made of insulating material, and secured thereto by clampsQ is a permanent horseshoe-magnet M, (or, if desired, an

electromagnet may be employed,) having detachable pole-pieces M and MThe polepieces are similar in construction, each being provided with afiat side,which makes a good magnetic joint with the side of magnet M,and a curved side, which partially surrounds moving coil C. The uppersides of the pole-pieces are grooved and mountedtherein are semicircularpieces of magnetic material P and P, constituting what I termauxiliarypolepieces. Referring to Fig. 2 it will be seen that the endsof the pieces near the zero of the instrument are high and graduallydecrease as the other end is approached, the object of which will behereinafter described.

Mounted for oscillating movement on a shaft having adjustablejewel-bearings J and Jis an armature or moving coil system, com--prising a body C, of magnetic material, surrounded by a coil or coilsC, wound and arranged in any suitable manner, the arrangement of turnsof the form shown in the drawings being similar to that on the armatureof a dynamo-electric machine, so that the lines of force in passing fromone pole-piece to the other will be cut by the turns of the coil whenthe latter is turned on its axis. Mounted on the coil-shaft is a smalltransverse piece of magnetic material I, which I have termed a magneticreturn-strip or auxiliary armature. This is situated with its endsdirectly over, but not touching, the curved pieces P and P, which aresecured to the pole-pieces, and while in the Zero position is closethereto; .but as the coil and strip I move away the distance graduallyincreases.

The tendency of any portion of a magnetic circuit to move to such aposition as to cause the total magnetic reluctance of the circuit to bea minimum causes the strip I to furnish at all times the necessaryrestoring torque, while the deflecting torque is furnished, as usual, bythe mutual action of the magnet and the suspended coil.

Extending from one pole to another is a non magnetic bridge L, andsecured to the bridge is an arm L which supports the lower adjustablebearing J. Connection with the moving coil is established by means offlexible connections B and B These are very light and, so far aspossible, neither assist nor retard the movement of the coil. It is tobe understood that these connections do not in any Way form theequivalent of the spring return commonly employed in measuringinstruments. Flexible connections B and B are respectively connected toinsulated terminals B and B which in turn are connected to binding-postsB and B, located on base A.

Mounted on the coil-shaft for movement across scale Z is a needle K,arranged to indicate by its position the angular deflection of coil 0.By varying the relation that the pole-pieces and magnetic return-stripbear to each other the indications on the dial maybe widely varied-thatis, the scale may be made so that the indications are close on the endsand open in the center, or it may be close in the center and open on theends.

Surrounding the working parts of the instrument is a suitable case S,having a glasscovered opening S, situated over the scale, and a tubularextension S surrounding the upper jewel-bearing. The action of myinvention is as follows: Current enters coil 0 from binding-post B byflexible connection B passes through the coil and out by way of flexibleconnection B to binding-post B.

This causes a rotation of the coil, the amount depending upon thecurrent passing, and the magnetic return strip I is moved into astronger part of the field and offers as it moves a certain oppositionto the movement of the coil. Increasing the current-coil C increases theangular movement until theinstrument has reached its capacity and needleK has traveled across the scale Z.

When my invention is applied to a voltmeter, coil 0 is made of highresistance and may be put in circuit with resistance to further limitthe current. For an ammeter the moving coil maybe put in shunt to adefinite resistance, so that the current flowing therein will bear adefinite relation to the total current, or When comparatively smallcurrents are to be measured it may all pass through the coil.

In ordinary commercial instruments the deflecting torque is dependentupon the strength of the magnet, while the restoring torque isindependent of the strength of the magnet and does not necessarily varywith any variation of the deflecting torque. The indications of theinstrument are thus liable to error from two sources, first, from anychange in the strength of the magnets, and, second, from any change inthe strength of the restoring torque.

In my improved instrument it will be seen that springs or theirequivalent-a gravity return-are entirely done away with, thus removingone common source of error, while any change in the deflecting torquedue to a change in the strength of the field-magnets is balanced by acorresponding change in the restoring torque, since the deflectingtorque is caused by the action of the magnet M upon the coil C, whilethe restoring torqueis caused by the action of the same magnet M uponthe needle I. The instrument thus becomes selfcompensating for allchanges of field strength within certain very wide limits.

It often happens that when instruments are received by the customer theneedle will not return to zero. This may be caused by the pointer beingbent, by a change in the spring or gravity return, causing the movingsystem to be slightly changed from the position it occu pied while beingcalibrated. From the appearance of the instrument it is impossible totell whether the fault is entirely with the pointer or with the springor gravity return, or both. This necessitates recalibration, usuallyobtained by returning the instrument to the factory, causing delay andexpense. With the arrangement shown if the needle does not point to zerowhen no current is flowing in the current-coil it shows at once that theneedle is bent, as the mag;- netic system always returns to the sameposition. It will be seen that this is of great importance, for itreduces the liability of damage to the instrument to a minimum, and theindicator can readily be set to the proper zero without recalibrationand with the cer- IIO tainty that this is all. the attention theinstrument requires.

\Vhat I claim as new, and desire to secure by Letters Patent of theUnited States, is- 1. The combination in an electric measuringinstrument of a magnetic motor mechanism for giving movement to theindicator away from the zero position, and a magnetic return mechanismopposing the motor mechanism and returning the indicator to the zero orinitial position, the motor and return mechanisms being subject to likechanges in the strength of the energizing magnet or magnets.

2. In an electric measuring instrument the combination of a magneticmotor mechanism giving movementto theindicator away from the initialposition, a return mechanism for returning the indicator to the initialposition, and means whereby any increase or decrease in the strength ofthe motor mechanism, under the action of a given magnetic fieldcorrespondingly in like ratio increases or decreases the strength of thereturn mechanism.

3. In an electric measuring instrument the combination of a motormechanism comprising an energizing-magnet and coil in inductive relationto the magnet, a return mechanism and means whereby any increase ordecrease in the strength of the energizing-magnet of the motor mechanismcorrespondingly increases or decreases the strength of the returnmechanism.

4:. In an electric measuring instrument, the combination of a magnet, amoving system in the field of the magnet, and magnetic return mechanismsituated in the same magnetic field for opposing the action of themoving system, and returning it to the initial position.

5. In an electric measuring instrument, the combination of a magnet, amoving system in the field of the magnet, and a piece or body ofmagnetic material also in the field of said magnet for returning themoving system to its zero or initial position.

6. In an electric measuring instrument, the combination ofafield-magnet, a coil mounted for movement within the field of the mzignet and adapted to be traversed by current from the circuit to bemeasured, and a piece or body of magnetic material for returning themoving system to zero, mounted on the same support as the moving coil,and Within the influence of the same magnetic field, so that, as thedeflecting force of the field-magnet changes, the returning force willbe correspondingly changed.

' 7. The combination of a magnetic motor mechanism and magnetic returnmechanism acted upon by the same field-magnet and so proportioned thatthe returning torque of the magnetic return mechanism Will bring theindicator to rest throughout the range of the instrument at pointscorresponding with and determined by the different ciirrent valuesmeasured.

8. In an electric measuringinstrument, the combination of a movablecoil, field-magnet poles arranged to embrace the moving coil, a magneticreturn mechanism, and polar extensions arranged to exert a changingattraction upon the magnetic return mechanism.

9. In an electric measuring instrument, the combination of a movablecoil, field-magnet poles arranged to embrace the movable coil, amagnetic return mechanism and auxiliary pole-pieces deriving their fluxfrom the main field-magnet acting upon the magnetic return mechanism.

10. The combination With a pivoted arma ture and a magnet, of anauxiliary armature connected to the main armature and so related theretothat the magnetic force acting upon the auxiliary armature will resistits movement and consequently the movement of the main armature fromnormal position, substantially as set forth.

In Witness whereof I have hereunto set my hand this 29th day ofDecember, 1897.

ELIl-IU THOMSON. Witnesses:

M. L. THOMSON, MINNIE F. SWARTHOUT.

